OVERALL ABSTRACT Mast cells and eosinophils are essential effector cells in both acute and chronic allergic inflammatory responses. The overall goal of this Program is to exploit eosinophil and mast cell Siglecs (sialic acid binding, immunoglobulin-like lectins) to prevent or treat immediate allergic reactions and chronic allergic inflammation. The overarching hypothesis is that specific mAbs, endogenous tissue glycans, or synthetic ligand analogs, can specifically and selectively engage complementary glycan binding Siglecs (CD33, Siglec-6, and Siglec-8) on eosinophils and/or mast cells to prevent or limit IgE-dependent and IgE-independent eosinophil and mast cell- related allergic responses. This application focuses on the role of three mast cell and eosinophil Siglecs that we hypothesize provide ideal targets for dampening allergic effector cell responses in a variety of acute and chronic allergy-related disorders. Dr. Bruce Bochner will serve as the PI of Project 1 (Defining Siglec-6 and Siglec-8 function on effector cells of allergic diseases as well as Core A (Administration) and Core B (Human mast cell and tissue acquisition core). Dr. James Paulson will serve as the PI of Project 2 (Siglec-targeted nanoparticles for treating mast cell mediated allergic disease) while Dr. Ronald Schnaar will serve as the PI of Project 3 (Human siglec ligands control mast cell and eosinophil mediated inflammation). This is a Program team with a proven track record of productivity and synergy. This application includes projects that examine Siglec/eosinophil/mast cell pathways from various perspectives including pharmacology, biochemistry, cell signaling, glycobiology, and cellular and molecular biology, using in vitro experimentation and in vivo humanized models. The majority of the proposed research utilizes human material including primary human cells and biologic samples, along with animal models involving humanized mast cell mice and novel knock-in strains of mice expressing human Siglecs of interest to provide more in-depth hypothesis testing on mechanisms and outcomes that cannot yet be assessed with human research. Each Project provides numerous elements of novelty ranging from human Siglec knock-in mice to development of unique reagents for targeting specific Siglecs to discovery of endogenous human tissue ligands for specific Siglecs.